In recent years, development of portable communication apparatuses is proceeding remarkably, and among others, downsizing and thickness reduction of mobile phones are in rapid progress. Other than mobile phones, downsizing and thickness reduction are in progress also for electronic apparatuses such as handy cameras (portable video cameras or the like), cordless telephones, laptop PCs (notebook sized PCs), and the like. Provided with a secondary battery mounted on an electronic apparatus main body, they are structured to be usable without plugging to an outlet, thereby increasing portability and convenience. However, the secondary battery has a limit in capacity and needs to be charged once every few days or every few weeks.
As charging methods, there are contact charging method and non-contact charging method. The contact charging method is a method to perform charging while the electrodes of a power receiving device and electrodes of a power feeding device are contacted with each other directly. The contact charging method is used in general because of its simple device structure. However, electronic apparatuses are becoming lighter in weight along with the downsizing and thickness reduction of electronic apparatuses in recent years, and thereby a problem arises such that the contact pressure between electrodes of the power receiving device and electrodes of the power feeding device becomes insufficient, thereby causing charging failure. The secondary battery is vulnerable to heat, and hence a circuit thereof should be designed to avoid over discharging and overcharging for preventing increase in temperature of the battery.
From this point, application of the non-contact charging method is considered. Conventional non-contact charging methods mainly use a nickel-hydrogen battery as the secondary battery, and hence have a problem that many of them take charging time of about eight hours. In this point, by emergence of high capacity secondary batteries such as Li-ion secondary batteries, application of non-contact charging methods is considered also for electronic apparatuses which consume relatively large power and require rapid charging, such as mobile phones and notebook sized PCs.
The non-contact charging method is a method such that a coil is provided in both the power receiving device and the power feeding device, and charging is performed using electromagnetic induction. Since the non-contact charging method does not require considering the contact pressure between electrodes, it is possible to supply a charging voltage stably without being affected by contact states of electrodes with each other. As a coil for a non-contact charging system, there is known a structure that a coil is wound around a ferrite core (refer to References 1, 2). There is also known a structure such that a coil is mounted on a resin substrate in which ferrite powder or amorphous powder are mixed (refer to Reference 3). However, the ferrite becomes fragile when processed to be thin, and hence is poor in shock resistance. A problem can easily occur in the power receiving device due to dropping of the device or the like.
For making the power receiving portion thinner corresponding to thickness reduction of an apparatus, there is adopted a flat coil formed by printing a metal powder paste in a spiral shape on a substrate. Further, structures are proposed in which the coupling between a flat coil (primary coil) on the power transmission side and a flat coil (secondary coil) on the power receiving side is intensified by a magnetic body (refer to Reference 4 to 6). The magnetic body (magnetic sheet) is used as a core material for intensifying the coupling between the primary coil and the secondary coil. However, as the transmission rate becomes higher, there arises a need for considering not only the coupling between the coils but heating of the surrounding parts.
Specifically, when the flat coil is used, magnetic flux passing through the flat coil is interlinked with a substrate or the like in the apparatus, and thus an overcurrent generated due to electromagnetic induction causes heating in the apparatus. Accordingly, large power cannot be transmitted, and a problem that the charging time becomes long arises. For example, when power transmission rate is raised ignoring the heating, carbon dioxide gas is generated in the Li-ion secondary battery, and there is concern that expansion and explosion or the like occurs. Accordingly, charging of a mobile phone by the non-contact charging system takes about 130% relative to the charging time by a contact charging device.
Li-ion secondary batteries used for mobile phones, digital cameras, portable game machines, portable audiovisual devices, and the like have five times or more charging capacity per unit time as compared to conventional nickel-hydrogen batteries. Therefore, when it is attempted to increase power transmission rate in the non-contact charging method, the problem of heating due to the overcurrent cannot be ignored. Thus, the power receiving devices to which conventional non-contact charging methods are applied have insufficient measures for the overcurrent generated by electromagnetic induction and heating based on this. The generation of overcurrent leads to generation of noise, and further becomes a factor to decrease charging efficiency.
[Reference 1] JP-A 11-265814 (KOKAI)
[Reference 2] JP-A 2000-023393 (KOKAI)
[Reference 3] JP-A 09-190938 (KOKAI)
[Reference 4] JP-U 58-080753 (Japanese Utility Model Publication)
[Reference 5] JP-A 04-122007 (KOKAI)
[Reference 6] JP-A 08-148360 (KOKAI)